1. Field of the Invention
The present invention relates to a device used in electrophotographic copiers for accurately exposing to light the edge zone of a photoconductive element to produce a clear, sharp margin on the edges of an image.
2. Description of the Prior Art
A typical device used in electrophotographic copiers for exposing an edge zone of a photoconductive element to light is described in U.S. Pat. No. 3,724,940. In this device, it is possible to set two or more different imaging ratios. The edge zone to be exposed to light can be adjusted for each imaging ratio. This device comprises a fixed light source and a pivotable light screen. By adjusting the pivotable light screen, the width of the edge zone required to be exposed to light can be varied for the different imaging ratios.
A disadvantage of this device is that a specific displacement of the light screen is not accompanied by a permanently fixed change in the dimensions of the edge zone required to be exposed. As a result, the operator must visually check that the light screen has been set to its correct position. To accomplish this, the copier in which this device is mounted is provided with a window through which the machine operator can check the width to which the edge zone has been adjusted by the movement of the light screen.
U.S. Pat. No. 3,901,593 describes an exposing device consisting of a box divided into two approximately equal parts by a light-tight wall. One part is for forming the top edge zone and the other part is for forming the side edge zones. One side of the one part has an aperture in the form of a long slit extending over the entire width of a photoconductive element, while the same side of the other part has two short slit apertures each extending over an edge zone of the photoconductive element. Shutter plates by means of which the size of the apertures can be varied are disposed in front of the short apertures. Mounted in the box behind the apertures are light sources. Also, light-scattering partitions are provided between the light sources and the short apertures. Apart from the fact that its construction is complicated, this device has the disadvantage that the shutters may jam or move stiffly due to fouling, making accurate adjustment impossible.
Similarly, U.S. Pat. No. 4,215,929 provides a very complicated device in which the image area is masked from exposure by overlapping polarized plates. By varying the overlapping area of the polarized plates, the width of the edge zones can be varied. Besides being very intricate and complicated, this device has the disadvantage that the polarizing plates must be moved in front of the control light in synchronization with the movement of the photosensitive element.
U.S. Pat. No. 3,799,666 discloses a permanently mounted shielding frame containing a light source which exposes the edges of an image to form a border. The disadvantage of this device is that the width of the edges cannot be varied. Moreover, this device can only be used with one image size. Similarly, U.S. Pat. No. 4,173,406 describes a device for exposing the edges of an image in which the size of the edges cannot be varied. If a different sized edge is desired, such as when the image magnification changes, an additional exposing device having its openings spaced to form the desired edge is required.
Thus, there is a need for an improved device for exposing the edge zones of photoconductive elements which overcomes the disadvantages mentioned above and which provides clear, sharp margins on the edges of an image.